According to the prior art, a spray gun, in particular a paint spray gun, at the head thereof has a paint nozzle which is screwed into the gun body. The paint nozzle at the front end thereof often has a hollow-cylindrical small plug, the material to be sprayed exiting from the front mouth thereof during operation of the spray gun. However, the paint nozzle in the front region thereof may also be conically designed. The gun head typically has an external thread by way of which an annular air nozzle having an air cap disposed therein is screwed to the gun head. The air cap has a central opening, the diameter thereof being larger than the external diameter of the small paint-nozzle plug, or of the external diameter of the front end of a conical paint nozzle, respectively. The central opening of the air cap and the small plug, or the front end of the paint nozzle, respectively, conjointly form an annular gap. The so-called atomizing air exits from this annular gap, said atomizing air in the above-described nozzle assembly generating a vacuum on the end face of the paint nozzle, on account of which the material to be sprayed is suctioned from the paint nozzle. The atomizing air impacts the paint jet, on account of which the paint jet is torn apart so as to form threads and tapes. On account of the hydrodynamic instability thereof, the interaction between the rapidly flowing compressed air and the ambient air, and by virtue of aerodynamic disruptions, these threads and tapes disintegrate so as to form droplets which are blown away from the nozzle by the atomizing air.
The air cap furthermore often has two horns which are diametrically opposed, in the outflow direction projecting beyond the mentioned annular gap and the material outlet opening. Two supply bores, that is to say horn air infeed ducts, run from the rear side of the air cap towards horn air ducts in the horns. Each horn typically has at least one horn air duct, each horn preferably having at least two horn air ducts, however. Each horn air duct on the external side thereof has a horn air opening from which the horn air exits. The horn air ducts or openings, respectively, are typically oriented such that the former in the exit direction point in the nozzle longitudinal axis towards the annular gap, such that the so-called horn air exiting from the horn air openings may influence the air or the paint jet, respectively, that has already exited from the annular gap, or the paint mist that has at least been partially created already. On account thereof, the originally conical cross section of the paint jet (round jet) or of the paint mist, respectively, at the sides thereof that face the horns is compressed and is slightly elongated in the direction that is perpendicular thereto. On account thereof, a so-called wide jet which permits a higher planar coating rate is created. Apart from deforming the paint jet, the horn air has the effect of further atomizing the paint jet.
So-called control openings may be incorporated into the front face of the air cap, so as to be radially outside the central opening. The air that exits from the control openings influences the horn air, the former in particular cushioning the impact of the horn air on the paint jet. Furthermore, the control air protects the air cap from contamination in that the former conveys paint droplets away from the air cap. Moreover, said control air contributes towards further atomization of the paint mist. The control air also acts on the round jet, causing a slight pre-deformation as well as additional atomizing here too.
Such a nozzle arrangement is above all suitable for use with a spray gun, in particular a paint spray gun, wherein not only paints but also adhesives or lacquers, in particular base and clear lacquers, both solvent-based as well as water-based, but likewise liquids for the food industry, wood-treatment agents, or other liquids may be sprayed. Spray guns may be classified in particular as hand-held spray guns and as automatic or robotic guns, respectively. Hand-held spray guns are used above all by tradesmen, in particular by painters, joiners and varnishers. Automatic and robotic guns are typically used in conjunction with a painting robot or a painting machine for industrial applications. However, it is readily conceivable for a hand-held spray gun to be integrated in a painting robot or in a painting machine.
The spray gun may have the following in particular: a grip, an upper gun body, a compressed-air connector, a trigger for opening an air valve and for moving the paint needle out of the material outlet opening of the paint nozzle, a round/wide jet regulator for setting the ratio of atomizing air to horn air in order for the paint jet to be shaped, an air micrometer for setting the spray pressure, a material-amount regulator for setting the maximum volumetric material flow, a material connector, paint ducts for conducting the material to be sprayed from a material inlet to the material outlet, compressed-air ducts, in particular wide-jet ducts for supplying the horns with air, and round-jet ducts for supplying the annular gap and the control openings with air, a suspension hook, and an analogue or digital pressure-measuring installation. However, said spray gun may also have other components from the prior art. The paint spray gun may be designed as a flow-cup spray gun, having a paint cup that is disposed above the gun body and from which the material to be sprayed flows substantially by way of gravity and by negative pressure at the front end of the paint nozzle into and through the paint ducts. However, the spray gun may also be a side-cup gun in which the paint cup is disposed laterally on the gun body, and in which the material is likewise infed to the gun by gravity and by negative pressure at the front end of the paint nozzle. However, the spray gun may also be as a suction-cup gun, having a paint cup that is disposed below the gun body, from which the material to be sprayed is suctioned substantially by negative pressure, in particular while utilizing the Venturi effect, from the cup. Furthermore, said spray gun may be designed as a pressurized-cup gun in which the cup is disposed below, above, or laterally on the gun body and is impinged with pressure, whereupon the medium to be sprayed is squeezed out of the cup. Furthermore, said spray gun may be a bucket gun in which the material to be sprayed is infed to the spray gun from a paint container by means of a hose or by way of a pump.
The above-described nozzle arrangement and spray gun have been successful for many years. The quality of the spray result depends to a large extent on the quality of the spray gun used. High-quality spray guns are manufactured with high precision to very tight production tolerances, since even deviations from the ideal dimension in the range of a few hundredths of millimeters may negatively influence the quality of atomization and thus the spray result. The quality of atomization is further determined by the accurate design of the so-called nozzle set. The nozzle set is typically composed of the air nozzle, the paint nozzle, and the paint needle. The air nozzle in turn is composed of the air cap and the annular air nozzle. The diameter of the needle tip, the internal diameter of the central opening in the air cap, of the horn air openings, and of the control openings, the angles of the openings or ducts, respectively, in relation to the central axis of the central opening, and the mutual alignment of the openings or ducts, respectively, are all relevant to the spray quality in particular.
A good atomization quality is particularly important in the application of clear and base lacquers (solid paints) to vehicles and vehicle parts. An inadequate spray quality has negative effects on the accuracy of the colour shade and the lustre of the coating in particular in the case of repair paintwork. Since the repainted vehicle part often is disposed directly next to a part having the original paintwork, any inaccuracies are clearly in evidence here. A complaint by the customer of the vehicle paint shop necessitates rework which is associated with a high expense in terms of time and costs.
It has been established in the context of spray tests that the quality of the coating does not depend only on the fineness of atomization but to a large extent also on the profile of the layer thickness of the coating across the length or height, respectively, of the spray jet, or of the spray pattern, respectively. A spray pattern is usually established in that paint or lacquer is applied by means of the spray gun, without moving the spray gun, from a specific distance, for example 15 cm to 20 cm, in front of a substrate, for example paper, a paper having a scale that is intended for establishing a spray pattern, or a sheet-metal panel. The spray duration is approx. 1 to 2 seconds. Alternatively, the spray gun may be moved by means of a device, in particular perpendicularly to the longitudinal axis of the wide jet, keeping a constant distance from the sheet-metal panel or paper. The shape of the spray pattern that has been generated in this way, and the size of the droplets on the substrate, provide a conclusion pertaining to the quality of the spray gun, in particular of the nozzles.
The layer thickness of the spray pattern may be ascertained pre or post drying of the spray pattern by means of methods known in the prior art, for example by means of layer-thickness measuring apparatuses, or the paint droplets and the size and position thereof is detected still during the flight towards the substrate, for example by means of a laser diffraction method.
A spray pattern as has been described above, across the length and the width thereof, does not have a uniform layer thickness. The central core of the spray pattern has a high layer thickness, the layer thickness generated outside the core being less. The transition in the layer thickness between the core and the external region is fluid. If the layer thickness is plotted across the length of the spray pattern, an initially flat ascent from the left to the right results, said ascent marking the external periphery of the external region. The core thickness increases relatively steeply in the proximity of the core, and in the ideal case remains substantially constant across the longitudinal profile of the core, that is to say that a plateau is displayed. The layer thickness drops relatively steeply at the periphery of the core, followed by a flatter descent towards the end of the external region. It has been demonstrated that a more uniform coating of improved quality may be generated the steeper the transition is between the core region and the external region, that is to say the steeper the profile of the layer thickness is across the length of the spray pattern when transitioning from the external region to the core region. During the painting procedure, the painter moves the activated spray gun in meandering tracks, wherein the tracks mutually overlap in a region of between 30% to 50% of the height of said tracks, that is to say that approximately the lower or the upper third of one track overlaps the upper or lower third of the preceding track, respectively. A core region of higher definition enables the painter to apply the core regions of the spray tracks during the painting procedure in as mutually adjacent a manner as possible such that a uniform overall layer thickness is created. However, the transition must also not be too steep since there is otherwise the risk of excessive coating, for example by inadvertently applying the double coating thickness, leading to so-called paint tears. The experiments have furthermore demonstrated that it is advantageous for the above-mentioned plateau to be as wide as possible, that is to say for the core region of the spray pattern having the maximum layer thickness to be as long as possible.